Measurements of Sn Thermally Enhanced Sputtering Yields at Nano-PSI

Capillary porous structure (CPS) based liquid metal divertors are currently being investigated as a possible alternative to the tungsten based solid plasma facing components (PFCs). The ability of CPS based technologies to withstand high heat fluxes (> 20 MW/m2) has been already demonstrated in linear devices as well as tokamaks. One of the key aspects of a liquid metal divertor is the erosion of the liquid metal with the subsequent contamination of the plasma. The liquid can be eroded by physical sputtering, evaporation and thermally enhanced sputtering. The absence of a theoretical model or detailed empirical data of Sn thermally enhanced sputtering prohibits reliable predictions of Sn erosion by fusion plasma. Especially in high density tokamak plasmas, thermally enhanced sputtering appears to be the dominant contributor to total erosion. To empirically evaluate the thermally enhanced sputtering yields an experimental campaign was conducted at the Nano-PSI device (T e = 0.3–0.8 eV, T i=5x10 18 m -2 s -1) with Sn surfaces exposed to homogeneous plasma of various ion species (Ar, Ne, H, He). The effect of ion impact energy on the sputtering yields was studied as well by biasing of the the liquid surface in range of -10 to -80 V. In case of Ar, Ne and He the Sn was exposed as a free-flowing surface and for H it was exposed in a stainless-steel capillary porous structure (CPS) to negate the observed H spitting of the free liquid surface. This work presents the measured thermally enhanced sputtering yields, with focus on the observed phenomena, such as plasma species and impact energy dependency.